The delivery of data content to users, which can include for example Internet content, media content, and voice communications, is provided through a distributed data network. FIG. 1 is an example data network diagram showing how service provider 10 provides data content to end users at their homes or office buildings 12, or wherever they may be located. Depending on geographical area to be covered, various intermediate nodes may be required to distribute the data content to the end users 12. In the example of FIG. 1, regional nodes 14 can function as a data delivery node to users in vicinity of the regional nodes 14, and can function as a repeater for redistributing the data content received from the service provider 10 to base stations 16. Base stations 16 can be located in a neighborhood to facilitate delivery of data content to the homes or buildings 12 located nearby. The base stations 16 can be configured to provide wireless services to users as well. It should be appreciated that the number of intermediate nodes between the service provider 10 and the end users 12 can be adjusted depending on the required geographical coverage of the data services.
The medium for carrying the signals representing the data content between the nodes, such as between service provider 10 and the regional node 14, between the regional node 14 and the base stations 16, and between the base stations 16 and the homes or buildings 12 are data cables 18, 20 and 22. These data cables can be electrical conducting cables made of copper, or they can be optical cables which carry data in the form of modulated laser light. It is well known that optical cables have a much larger data bandwidth than copper cables, and have the benefit of low signal loss over long distances. That being said, optical data transmission is still subject to various phenomena which can distort the optical signal, and must be compensated for in order to recover the transmitted data.
FIG. 2 is a simplified diagram of an optical data transportation link 30, which includes a transmitter 32 and a coherent receiver 34 connected to each other by an optical propagation channel 36. Each pair of nodes shown in the example data network diagram of FIG. 1 can have the optical data transportation link 30 presently shown in FIG. 2.
The transmitter 32 generates an optical signal comprised of two orthogonal linear polarization components (X and Y), wherein each component is comprised of two orthogonal phase components (in-phase I and quadrature Q) that have the same carrier frequency. The carrier frequency is an optical wavelength supplied by a laser with phase noise. The propagation channel 36 is comprised of optical filters such as cascaded WSS, fiber, amplifiers that are the sources of chromatic dispersion (CD), nonlinear phase noise, polarization mode dispersion (PMD), polarization dependent loss (PDL), polarization dependent gain, polarization rotation and optical white Gaussian noise.
The coherent receiver 34 is comprised of an integrated coherent receiver, photo detectors (PIN), analog to digit converters (ADC) and a DSP unit. The integrated coherent receiver 34 is the place where a local oscillator (LO), with a frequency that is closely matched to the transmitter laser, mixes with a propagated optical signal and splits it to four signals with each being a mixture of transmitted signals. The DSP unit is where signals are processed and data are recovered. Further details of all the above mentioned components are discussed later.
One of the problems with optical transmission is frequency wander, where a frequency shift in the base band signal occurs relative to the frequency at the transmitter 32. This is referred to as local oscillator frequency offset (LOFO), and the resulting signal at the receiver 34 has a frequency that is not exactly matched with that of the transmitter 32. The LOFO needs to be corrected at the receiver 34 in order to recover data in the optical signal. In some currently known systems, the LOFO can be as large as ±5 GHz.
Most known solutions follow a two-step approach for determining the frequency offset of the received signal. First a coarse frequency offset estimator (FOE) can estimate and correct LOFO to less than ±1 GHz estimation error. Then a fine estimation is executed to determine the final LOFO with an estimation error of less than 10 MHz. However, most known fine LOFO estimator solutions are very complex and thus costly to implement, vulnerable to different types of impairments which increase the estimation error beyond an expected threshold, or are only effective for specific modulation formats such as BPSK and QPSK but not for other formats which must also be supported by the same product.
While some of the above mentioned techniques can be used, they may not be effective for newer systems capable of increased bandwidth and increased modulation. In other words, application of the currently known techniques for frequency offset estimation could result in a very slow data recovery time at the coherent receiver 34, or worse, the coherent receiver 34 may simply fail.
It is, therefore, desirable to provide a fine LOFO estimator system and method that is simple to implement, accurate in fine frequency offset estimating, and universal such that it is compatible with all systems.